The invention relates to a process and/or a method for producing urea pellets having the features mentioned in the preamble of claim 1.
In motors and particularly in the motor vehicle industry it is problematic to clear the exhaust of diesel engines, for example in motor vehicles, from nitrous oxides (NOx, denitrification). This serves for environmental protection and only by doing so can the regulations regarding Euronorm 5 and 6 be fulfilled. Ammonia is necessary for the denitrification of such exhausts using SCR-technology (selective catalytic reduction). It is rather impractical to carry bottled ammonia along in the vehicles and impossible for safety reasons. An easily handled ammonia carrier was demanded and has been found in the form of urea. Urea provides the ammonia necessary for the catalytic removal of nitrous oxides from exhausts of diesel engines. In order to allow this urea to be used for the denitrification of diesel exhaust in the interest of high performance it must be provided in a concentrated form, thus as a granulate of solid matter, which in comparison to possibly also aqueous solutions is advantageous in that no heating of the tank is necessary to avoid freezing. Due to the need for a precise dosing of urea granulates in respective devices to release ammonia from urea, a high quality standard of urea granulates is necessary. In particular, grain size, density, and solidity are to comply with certain specifications.
The use of urea granulates is known in the fertilizer industry. Here, prilling towers, fluidized bed arrangements, granulators, etc. are the state of the art for their production.
From DE 29 08 136 a method is known to produce urea granulates in a fluidized bed, in which solid, powdered urea is placed onto a fluidized bed and serve as seeds for the urea granulates to be produced. In the fluidized bed these seeds are sprayed with a liquid comprising the material to be included in the urea seed particles. The liquid adheres to the urea particle seeds and the material granulates thus produced are dehydrated and solidified in the fluidized bed.
This method is disadvantageous in that an agglomeration of urea seeds cannot be avoided and a homogeneous structure of the product to be produced cannot be achieved by spraying so that a final material develops having different and inhomogeneous qualities and compositions.
From DE 197 24 142 A1 the production of granulated composite fertilizer is known using melt-suspensions. The melt-suspensions comprise a mixture of melted urea and fine-grained granulated inorganic salts, which are added at temperatures above the melting point of a fluidized bed known per se with a classified outlet of the finished product via the bottom of the fluidized bed, and which are injected into the fluidized bed via pressurized nozzles from the bottom and in the direction of the flow of the fluidized medium. The temperature of the injected whirling air, creating and maintaining the fluidized bed and accepting the crystallization heat of the introduced melt-suspension, is below the melting temperature of the melt so that a defined solidification and granulation of the melt-suspension in the fluidized bed occurs at temperatures ranging from 95 to 105° C. forming granulates, in which 95% of the material shows a grain size ranging from 1.6-5 mm and shows a biuret content <0.8 M-%. Biuret is a decomposition product of urea, which primarily develops during heating under ammonia separation.
From DE 31 17 892 A1 a spouted bed apparatus is known for the production of granulates, in which a liquid is inserted into the flow of solid matter of a spouted bed granulator. The spouted bed granulator has a circular cross-section, with its bottom part being embodied conically constricted. A gas channel, in which a nozzle for injecting the liquid is arranged, mouths into the central conical part of the spouted bed granulator. A respective gas is fed through the gas channel to maintain the fluidized bed. The centrally introduced gas entrains the liquid inserted by the nozzle and a portion of the material located in the jet bed granulator. Here, a more or less defined spray zone develops, in which the material particles can come into contact with the liquid. The sprayed material is reintroduced to the spouted bed via the cone-shaped floor so that a particle circuit develops. When an appropriate granulate size has been reached the particles are removed from the spouted bed granulator.
In this type of spouted bed granulator, the introduction of the gas to create the fluidized bed and the liquid to be introduced enter at a common place in the lower part of the spouted bed granulator. The particles falling back are slowed down by the vertically flowing fluidization means (air), are entrained, and deflected upwards. Here, in particular in case of larger particle amounts, pulsation can develop as well as inhomogeneous particle concentrations in the proximity of the nozzles, resulting in an inhomogeneous coating of the particles. The relative large open area of the gas entry in form of a circular ring tends to form air hanks, and thus an inhomogeneous flow. In order to achieve stable processing conditions the device must be operated at higher air flow speeds, thus the fluidization conditions cannot be adjusted freely, solely depending on optimum processing conditions. The excessive gas speeds yielded here in the proximity of the nozzles lead to the development of dust by spray drying and/or spray crystallization and the developing dust adheres to other particle surfaces during the spraying process and reduces the surface quality and the homogeneity (e.g., sphericity) of the product. For these reasons a homogeneous wetting of the material particles to be treated is hardly realized or not at all. Some material particles are sprayed with too much liquid, others with too little, so that a final product cannot be achieved that has an even grain size and a homogeneous material structure. Additionally the arrangements are only suitable for the granulation of small material throughputs, in larger throughputs problems develop with regard to create and maintain the fluidized bed.
Additional publications, such as DE 31 17 892, DE OS 102 52 734, DE 693 10 629, EP 0 026 918, and EP 1 136 465 include methods, in part also for the production of urea pellets, which are however unsuitable to produce urea pellets meeting the necessary high requirements set by the automotive industry, in particular to achieve a very narrow range of grain sizes, high sphericity, a smooth surface, and a low defined residual moisture.
All known methods for producing urea granulates have the common disadvantage that the fabricated bulk product, i.e., urea granulate, complies insufficiently or not at all with the specifications set for the exhaust denitrification of diesel engines. The granulates are insufficiently spherical, have a wide range of grain sizes, and fail to have a smooth surface.
On the other hand, from DE 103 22 062 A1 a method and a device are known for the application of liquids in a solid flow of a spouted bed apparatus. Here, the supply air necessary to form a solid flow is introduced via a gap located in the lower area in the axial direction of the reaction chamber into a solid flow in an approximately axial direction of the reaction chamber of the spouted bed apparatus. The liquid is inserted into the material flow via one or more single or multi-way nozzles at one or more sites. Here, the flow conditions in the spray area can be adjusted so that the liquid can be applied to the material flow in a targeted and controlled manner. The finished product developing here is characterized in an almost homogeneous grain size having identical material features. Here, different final products can be produced by spraying pure liquids, solutions, melts, or the like by one or more single or multi-way nozzles into the material flow.